Hypertrophic cardiomyopathy in cats

Cats are creatures of mystery – notoriously enigmatic, aloof, independent. Although many pet cats can also be incredibly affectionate, loving and loyal, they still unfortunately are very good at hiding signs of illness. 

One of the most common conditions that can easily go unnoticed is hypertrophic cardiomyopathy (HCM), a form of heart disease which, sadly, is often fatal. Hypertrophic cardiomyopathy can even be a cause of sudden death in young cats. But is there anything that can be done to try to diagnose it earlier and treat it? And once it reaches the terminal stages, how do you make the kind decision to end their suffering? 

Pathophysiology of hypertrophic cardiomyopathy 

The heart is composed of muscle arranged into four chambers – two lower ventricles and two upper atria. Between these chambers are valves which direct the flow of blood and ensure it can only move in the direction it’s supposed to. In hypertrophic cardiomyopathy, the wall of the left ventricle, which is responsible for pumping the blood around the body, thickens around the chamber. In time, this will put extra pressure on the mitral valve between the left atrium and left ventricle, which can result in failure of the valve and the backflow of blood into the atrium from the ventricle. Because of this, the left atrium will enlarge, and this combination of failures can lead to heart failure.  

Unfortunately, because of this turbulent blood flow, cats with HCM are at risk of developing blood clots which can be fatal, causing sudden death or irreversible blood clots, leading to blockages called thromboembolisms.  

Symptoms of hypertrophic cardiomyopathy 

In the early stages of the disease, and in mild cases, there may be few outward symptoms, or even none at all – these are classed as subclinical cases. Up to a certain point, the heart can cope with the structural changes, but there will likely come a time when it can no longer function as normal. It is then that the cat can start to show symptoms of congestive heart failure as explained above. The main signs that you may begin to see are: 

• An increased respiratory rate – the normal respiratory rate for a cat is between 16 and 30 breaths per minute. This will increase when they are at exercise or at times of stress, but these are normal expectations. Because of this, we advise to measure the respiratory rate when the cat is sleeping and not dreaming. If, even at this time, the respiratory rate is above 30bpm, it could indicate a problem. An increase in respiratory rate due to congestive heart failure occurs because of the extra stress and strain on the heart which is having to work harder. It can also be because of fluid build-up within the lung tissue making breathing harder.  

• Extra effort to the breathing – similar to the increase in respiratory rate, an increased effort to the breathing is often because the lungs can fill with fluid meaning it is harder for the cat to get air in. Fluid can also build up around the lungs, within the chest cavity. If a cat is breathing with their mouth open so it looks like they’re panting like a dog, it can be a sign of severe respiratory distress and should prompt urgent veterinary investigation.  

• Lethargy – because of the extra energy required to try to overcome the clinical effects of heart failure, plus often reduced oxygen levels, cats will typically become quite lethargic and seem to tire easily.  

• Inappetence – heart failure can make cats feel really quite poorly, so they will just generally become quieter and are likely go off their food too. Inappetence is a very non-specific sign and could also be a symptom of many other conditions.  

• Sudden onset paralysis – many owners don’t realise, but an acute onset paralysis, often of the hindlegs, can be a sign of a blood clot caused by HCM. They often think their cat has been hit by a car, but a lack of pulses to the back legs, cold pale feet and extreme pain are all signs that this is cardiac, not trauma. In many cases, this is non-recoverable.  

How is hypertrophic cardiomyopathy diagnosed? 

Quite often, by the time a cat with HCM sees a vet, they will already be showing clinical signs and this, in combination with clinical examination findings, will increase the suspicion for heart disease. When carrying out a clinical examination, the vet will be listening for things like an abnormal heart rhythm, a heart murmur, muffled heart sounds, crackling on the lungs and quiet lung sounds. It is likely that the heart rate will be elevated, and the cat may have poor pulses. In some cases, abnormal findings may be detected during a routine health check, with clinical signs either not being present or not been noticed by the owners.  

In either situation, but especially if symptoms are evident, the best way to find out if there is heart failure and whether it’s caused by hypertrophic cardiomyopathy is to carry out diagnostic imaging, usually with echocardiography (an ultrasound scan of the heart), an ECG (heart trace) and possibly a chest x-ray.  

In cases of hypertrophic cardiomyopathy, an echocardiogram will show the thickened heart muscle of the left ventricle and, for more advanced cases, the abnormal blood flow within the heart and through the mitral valve. Fluid within or around the lung tissue can also often be seen on a scan. An ECG will provide further evidence of any electrical dysfunction within the heart and a chest x-ray can confirm the presence of lung fluid and look for any other concurrent pathology.  

Treatment of hypertrophic cardiomyopathy 

Hypertrophic cardiomyopathy and any associated heart failure cannot be cured, only managed. Treatment is aimed at alleviating any excess fluid build-up, controlling the heart rate, improving the heart function and preventing clots. Currently, there is no evidence that starting treatment before the onset of clinical signs will prevent or slow down the disease progression, however it can be a good idea to start medication to try to prevent clots from forming, as soon as HCM is diagnosed.  

Prognosis for hypertrophic cardiomyopathy 

The prognosis for cats with hypertrophic cardiomyopathy is variable. However, many cats can continue for years without any clinical signs. Once clinical signs develop and treatment has begun, owners need to be aware of signs of deterioration, as cats are good at hiding when things are wrong. There will likely come a point when the treatment can do no more and the clinical symptoms cannot be controlled. In order to prevent suffering, owners may need to make the decision to euthanise their pet. This is never easy, but the number one thing to consider is the animal’s quality of life. 

If you are worried about your cat’s quality-of-life and need help making the final decision, our questionnaire can help you assess whether they are still able to enjoy life. You can also call our trained advisors, who are able to guide you through the process.  

Further resources: 

https://www.msdvetmanual.com/cat-owners/heart-and-blood-vessel-disorders-of-cats/acquired-heart-and-blood-vessel-disorders-in-cats#Disorders-of-the-Heart-Muscle-(Cardiomyopathy)_v3243314 

https://heartvets.co.uk/wp-content/uploads/2021/09/HCM-Info-sheet-HeartVets-1.pdf 

 https://cloud9vets.co.uk/is-it-time/